The headlights used in the first vehicles were very rudimentary; basically, they were lanterns that used a candle inside a glass housing to light up the road. Later they advanced to gas burning lanterns, and kerosene lamps, like the ones Coleman still makes for camping. It is almost hard to fathom today, but then these cars were only traveling at a jog. Thomas Edison, the inventor of the incandescent light bulb, and is credited with the first long-lasting headlamps for cars that could handle the vibrations and most cars today still use lights based off his original designs.
Simply put, a common light bulb has a filament and when electricity is applied, the filament gets very hot and reaches a state of incandescence (glows bright red/orange/yellow). Incandescent light bulbs are not very efficient - about 5%-10% goes to making light and the rest makes heat.
The newer LED and Xenon HID lights use a different system, and are much more efficient, producing more light, and less heat, from a given amount of electricity.
Incandescent light bulbs typically put the filament in a vacuum to preserve it, but as used in car headlights, they are filled with special gases in the halogen family (usually iodine or bromine). Halogen reacts to the hot filament to help create a larger amount of light that is much whiter in color, because the filament burns hotter.
Burning hotter allows them to five off more light, but it also means the halogen bulb must be made of special glass and handled carefully so as not to leave fingerprints on it that can cause a hot spot. One fingerprint on a halogen bulb can cause it to fail within minutes once it is turned on.
Xenon HID headlights are not the same as a halogen unit. The HID light bulb (which stands for High Intensity Discharge) is filled with Xenon gas, and there is no filament. Instead, a spark is created inside the HID bulb between two electrodes, but requires a much higher voltage in order to make the jump. HID headlights create a much more powerful light - measured in thousands of lumen - for a given amount of wattage used.
A ballast is used to step up the 12 volts from the car's electrical system to 24,000 volts of electricity, which creates a potent spark and ignites the Xenon gas mixture. Once the spark is lit, the voltage is dailed back down, but still in the thousands of volt range.
Halogen does not emit as much light as a HID light, and road vibrations will cause the halogen incandescent filament to slowly decay and burn out.
But, halogen headlights are dirt cheap because they have been made for more than 50 years. While there are some HID retrofit kits available, halogen and xenon bulbs need a different reflector and mount in order to provide the best light; it is not just a matter of changing bulbs.
There are many advantages to the Xenon HID headlight:
At night, the Xenon HID headlight projects a beam of light that projects further ahead, while also illuminating the sides of the road better. Remember, the better you see the more time you have to react.
The biggest drawback to HID lights are the expense of them, which is why they are only on higher priced cars, or the top trim level of more affordable cars. While the price is coming down on them, they may soon be eclipsed by widely available, less expensive LED units.